Mitigating noise and obi in rfog networks

ABSTRACT

A bi-directional optical transceiver includes multiple single mode optical ports and a multi-mode optical port. A multi-mode optical combiner combines single mode optical signals received at the single mode optical ports into a multi-mode optical signal at the multi-mode optical port. Each single mode optical signal has a distinct optical mode that does not interfere with the optical mode of the other single mode optical signals. A photo detector detects a total optical power of the plurality of single mode optical signals in the multi-mode optical signal. An amplifier is coupled to receive an output of the photo detector.

BACKGROUND

Optical signal sources have a significant phase noise. When two opticalsources are combined, an additional signal is produced in a noise bandaround a center frequency, (w1-w2). If the frequency range of thisunwanted signal band overlaps with wanted signals, the signal to noiseratio of the wanted signal may be severely impacted. This is calledoptical beat interference (OBI) and is a practical problem, particularlyin cable television (CATV) return systems, where multiple opticalsignals are combined on a single detector.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, the same reference numbers and acronyms identifyelements or acts with the same or similar functionality for ease ofunderstanding and convenience. To easily identify the discussion of anyparticular element or act, the most significant digit or digits in areference number refer to the figure number in which that element isfirst introduced.

FIG. 1 is an illustration of an embodiment of a conventional RFcommunication system.

FIG. 2 is an illustration of an exemplary RF communication system withreduced RF noise in the reverse communication path.

FIG. 3 in an illustration of an exemplary application of low squelch ina system with two reduced size receiver groups of thirty two consumerdevices apiece.

FIG. 4 is an illustration showing an example of the application of lowsquelch for a larger number of consumer device groups, in this caseeights groups, each comprising thirty-two transmitting devices.

FIG. 5 is an illustration of an embodiment of low-squelch logic.

FIG. 6 illustrates an exemplary system for communicating radio frequencydata signals over an optical physical transmission path, e.g. RFoG datacommunication system.

FIG. 7 illustrates a power verses frequency distribution at an exemplaryreturn receiver in an RFoG communication system without the presence ofunacceptable OBI.

FIG. 8 illustrates a power verses frequency distribution at an exemplaryreturn receiver for return signals comprising a significant amount ofOBI.

FIG. 9 illustrates an RFoG system in which an exemplary return receiverdetermines if an OBI event has occurred and provides an OBI detectionsignal to an RF signal generator that modulates a forward path laser.

FIG. 10 is an illustration of an embodiment of logic flow for atransmitter determining whether or not to make a wavelength shift inresponse to an OBI event.

FIG. 11 is an illustration of an embodiment of a system to make awavelength shift in response to an OBI event.

FIG. 12 illustrates an example of simulated OBI occurrences fordifferent parameters such as channel activity, number of users, andgroup sizes.

FIG. 13 illustrates an embodiment of logic flow, by way of which returntransmitters learn their associated group ID.

FIG. 14 illustrates an example of how forward signal injection may beaccomplished in the return receiver itself.

FIG. 15 illustrates an embodiment in which the OBI detection signal isinjected into a main forward signal before the main forward signal isdistributed over different return receiver groups.

FIG. 16 illustrates an embodiment in which the OBI forward signal isprovided as an RF signal to a forward optical transmitter that isproviding the main forward signals of the RFoG system.

FIG. 17 illustrates an exemplary RFoG WDM system.

FIG. 18 illustrates an exemplary system in which an SOA at the returntransmitter is driven by a pre-distorted signal to compensate fornon-linearity.

FIG. 19 illustrates an exemplary multimode coupler that may be used tocombine multiple single mode fiber inputs with optical frequenciessufficiently close that optical beat interference could occur.

DETAILED DESCRIPTION

References to “one embodiment” or “an embodiment” do not necessarilyrefer to the same embodiment, although they may.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, refer to this application as awhole and not to any particular portions of this application. When theclaims use the word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list and anycombination of the items in the list.

“Logic” refers to machine memories (e.g., memory circuits or magnetic oroptical media) and/or circuits comprising control/procedural signals,and/or settings and values, that may be applied to influence theoperation of a device. Magnetic media, random access memory chips (bothvolatile and nonvolatile), electronic and optical circuits, electricaland optical memory (both volatile and nonvolatile), and firmware areexamples of logic physical structure. In general, logic may comprisecombinations of software (embodied in machine readable media and/ormemories), hardware circuits, and/or firmware.

Those skilled in the art will appreciate that logic may be distributedthroughout one or more devices, and/or may be comprised of combinationsof instructions in memory, processing capability, circuits, and so on.Therefore, in the interest of clarity and correctness logic may notalways be distinctly illustrated in drawings of devices and systems,although it is inherently present therein. The techniques and proceduresdescribed herein may be implemented via logic distributed in one or morecomputing devices. The particular distribution and choice of logic is adesign decision that will vary according to implementation.

Noise Reduction and OBI Mitigation Techniques

Described herein are various techniques to reduce noise, andparticularly OBI, in optical RF communication systems. The describedtechniques include:

1. OBI Mitigation Techniques

2. Optical Combining

3. RF Combining

4. RF Combining with Lo Squelch

5. RF Combining with Hi Squelch

6. RF Combining with High and Lo Squelch

7. Multimode Optical Combining

8. OBI Homogenization

9. Super Luminescent Laser Diodes as the light source

10. Highly coherent lasers to minimize interaction

11. High line width, high chirp lasers to minimize the effects of OBI

12. By a deliberately high OMI on a single channel

13. Use of high line width FP lasers

14. Use of a broadband source with a reflective amplifier

The following is a summary of communication techniques in an RFoGsystem, and their effect on RF noise and OBI.

1. Optical Combining: Combining 256 ONUs optically

-   -   No additional RF Noise at the receiver; regular occurrence of        OBI.

2. RF Combining: Combining eight sets of 32 cluster ONUs

-   -   Additional RF noise in the reverse transmission receiver; less        occurrence of OBI than in (1)

3. RF Combining with Lo Squelch

-   -   Less additional RF noise in the receiver than (2); less        occurrence of OBI than (1) or (2)

4. RF Combining with Hi Squelch

-   -   Additional noise in the receiver; less occurrence of OBI than        (1)

5. RF Combining with High and Lo Squelch

-   -   Less additional noise in the receiver than (4); less occurrence        of the OBI than (4)

6. Multimode Optical Combining

-   -   No Additional Noise in the Receiver; lesser occurrence of OBI        than (1)

RFoG Low Squelch

FIG. 1 is an illustration of an embodiment of a conventional RFcommunication system. A large group of end user devices, in this example128, communicates information in a reverse direction to a cable modemtermination system (CMTS). The signals from all 128 end user devices inthe return direction are combined and communicated to a reverse receiverand from there to the CMTS.

FIG. 2 is an illustration of an RF communication system with reduced RFnoise in the reverse communication path. End user equipment is arrangedinto smaller groupings, in this case thirty-two units per group, andreturn signals from each group are combined. Communications from eachgroup in the reverse direction are combined and provided to a returnreceiver for the group.

Communications from the return receivers are combined upstream from thereceivers before being provided to the CMTS. In one embodiment, a lowsquelch feature is applied at each individual receiver in a system suchas illustrated in FIG. 2. The low squelch feature is activated when thecombined signal power at the receiver indicates a quiescence state forthe subscriber devices in the communication group. A quiescence state isnot the same as no signal power, because there will be some noise signalpower at the receiver for the group even when none of the consumerdevices are transmitting upstream information. However, when the powerprofile indicates that all devices in the group are quiescence in termsof communicating information in the reverse path, the receiver shuts offits output to the RF combiner, producing no signal power to thecombiner. Without the low squelch feature, the receiver would becommunicating some signal power to the RF combiner even though none ofthe consumer device transmitters in its group are communicating upstreaminformation. This quiescence signal power provided by the receiver tothe RF combiner results in noise on the reverse path even when there isno actual communication of upstream information by consumer devicetransmitters in the group.

FIG. 3 in an illustration of the application of low squelch in a systemwith two reduced size receiver groups of thirty two consumer devicesapiece. Each consumer device group includes both optical networkingunits and devices which communicate via RF over coaxial cable. Thegraphic at the bottom of the figure shows results of applying a lowsquelch to the two return receivers for the two consumer device groups.The NPR (Noise Power Ratio, a measure proportional to the signal tonoise ratio available in the channel) is shown on the Y axis as afunction of signal power on the X-axis. With low-squelch the noise ofthe two receivers does not add up resulting in an improved signal tonoise ratio. At high signal power the signal to noise ratio is reduceddue to distortion in the transmission channel, this is not affected bythe low-squelch.

FIG. 4 is an illustration of an embodiment showing the application oflow squelch for a larger number of consumer device groups, in this caseeights groups, each comprising thirty-two transmitting devices. Theconsumer devices comprise return transmitters that are opticallycombined in groups of 32 each and provided to optical receivers 1.8located, for instance, in a hub or in a node. Each of these receiverscomprises a low-squelch circuit such that output is quiet when theirinput is idle. The combined output of these receivers is provided to atransmitter. The transmitter then sends the signals back to the headendvia a fiber (for instance 20 km) and the signals are received at areceiver which provides RF signals to a CMTS (Cable Modem TerminationSystem).

FIG. 5 is an illustration of an embodiment of low-squelch logic. Theoutput of a return RF receiver is ON if Detector 1 OR Detector 2 outputsHIGH. The output of a return RF receiver is OFF (squelched) if Detector1 AND Detector 2 outputs LOW. In one embodiment, an RC time constant(see FIG. 5) is 5 us, which determines the quiescent time that willresult in Rx output squelch.

Overview of Hi Squelch Techniques

A hi squelch feature to disable an upstream (reverse transmitter) outputmay be is implemented in various ways. For example, hi squelch may beimplemented using RF detection in a non-communication band. A high passfilter is located in front of a reverse path (upstream) RF detector in areverse receiver. If the RF detector detects RF signals, hi squelch isput into effect, and the receiver output is disabled.

Hi squelch may be implemented using optical power and RF powerrelationships. RF power is received in proportion to the optical powerreceived. When multiple transmitters are transmitting to the sameoptical receiver at the same time, the optical power at the receiver isincreased and the total received RF power is increased in proportion. Ina well balanced system, when two transmitters transmit at the same time,the optical power at the receiver is increased by 3 dB and the RF poweris also increased by 3 dB at the detector when there is no OBI. When theOBI occurs, the amount of RF power received is in excess of the opticalpower incident on the receiver. In one embodiment this mismatch triggershi squelch. False positives may occur when consumer devices (e.g. cablemodems) are ranging or when a cable plant experiences ingress. If thereis high or unexplained increase in RF power without any commensurateincrease in optical power, the changes are attributed to ingress orcable modem ranging and not symptomatic of the OBI. Therefore in someapplications hi squelch is not put into effect in this circumstance.

A combination of these two techniques yields accurate hi squelchoperation, where the squelch reaction time may be similar to the squelchreaction time for lo squelch operation (e.g. 5 microseconds in someembodiments).

Hi squelch effect may vary as a function of the number of independentreverse communication channels. The effect of hi squelch becomes moreimportant as the number of independent RF reverse channels increases. Itis estimated that the benefit of a hi squelch in an always ON eightchannel system is an up to four times reduction of OBI events and in acomparable four channel system, the improvement is around a two timesreduction in OBI.

OBI homogenization is a technique to smooth the effects of OBIoccurrence at the customer devices. When OBI occurs in a cluster of ONU,the OBI occurrence can be absent for prolonged periods of time and thenoccur during prolonged periods of time as return transmitter wavelengthsslowly drift as a function of temperature. Thus even if the average rateof OBI occurrence is low, when it occurs it can occur over a prolongedperiod of time. This is referred to as “lumpy”. OBI lumpiness resultswhen there are two lasers transmitting at wavelengths very close to eachother. This lumpiness degrades customer experience as it does not simplyslightly reduce data throughput, it may substantially eliminate datathroughput for noticeable amounts of time. One way to decrease thelumpiness and generate homogeneity in the OBI is to have the wavelengthsof the ONUs vary deliberately so that no two combinations of the ONUsexperience excessive OBI. This is achieved by making sure that an ONUuses a different wavelength setting whenever it turns on, by acontinuous changing of the wavelength at rest or while bursting. Thesehomogenizing techniques result in a more equitable distribution of theOBI, and not an elimination of OBI in general.

Another manner of homogenizing OBI is to set up multiple bias currentstates around the nominal bias current state. For example, if the laseris operating at 70 mA above threshold, set up five states ±5% away fromthe 70 mA and ±10% away from the 70 mA: states of 63, 66.5, 70, 73.5, 77mA. The laser generally moves at 1 GHz/mA so the laser wavelength is nowspread over ±7 GHz, enough to induce a homogeneity in OBI. Anotherhomogenizing technique is to set up a change in the laser temperaturethru a proximately placed resistor over ±5 C with a slow slew rate. Thelaser wavelength varies by 0.1 nm/C resulting in effectivelyhomogenizing the OBI occurrence.

OBI Mitigation In RFoG Systems

One approach to OBI mitigation involves allocating transmitterwavelengths such that OBI is reduced or eliminated. Another techniqueinvolves controlling the nature of the transmitter output spectrum suchthat the effect of OBI is reduced or eliminated.

FIG. 6 illustrates an exemplary system for communicating radio frequencydata signals over optical physical transmission path, e.g. RFoG datacommunication system. A main forward transmitter provides an opticalsignal to an amplifier (e.g. an erbium fiber doped amplifier EFDA) thathas multiple outputs from a splitter. Each splitter output goes to ared/blue (RB) combiner/splitter that combines the forward signals onto afiber that also carriers return signals. Typically the forward opticalsignal is in the 1550 nm range and the return optical signal is in the1310 nm range or at other wavelength ranges. The return signals arereceived by return receivers, one for each group of users. The fiber isprovided to a splitter that distributes the forward optical receivers tothe end users (RTx), which each comprise a receiver for the forwardwavelength and a transmitter for the return wavelength. The transmittersprovide return signals. When multiple transmitters are on at the sametime, OBI events can take place when transmitter wavelengths coincide.

Automatic Wavelength Allocation

One OBI mitigation technique applies a return receiver to detect that anOBI event has occurred. The receiver detects an RF spectrum that extendsbeyond a predetermined frequency band, indicating an OBI event.

FIG. 7 illustrates a power verses frequency distribution at a returnreceiver in an RFoG communication system without the presence ofunacceptable OBI. The power of the return signal are confined to bandof, for instance, 5-42 MHz.

FIG. 8 illustrates a power verses frequency distribution at a returnreceiver for return signals comprising a significant amount of OBI. OBIappears as a wideband signal exceeding several 100 MHz and comprises asignificant amount of signal power outside the return band.

Logic of the return receiver monitors the spectrum of the return signalsto determine if an OBI event has occurred. When significant power isdetected outside of an expected frequency range, it determines that anOBI event has occurred. The receiver may also detect an OBI event bymonitoring the average RF power. In many OBI events, the average powerexceeds normal power levels such that an OBI event may be detected bysetting a threshold at a power detector. A hi-squelch activator and/orout of band power detector may be applied to squelch the receiver outputwhen an OBI event occurs.

OBI detection may be based on average power, although preferably it isbased on detecting power outside the signal frequency band. The latterdetection method may be more sensitive to OBI events when the OBI noisepower is significantly below the signal power, such that the OBI cannoteasily be detected as an increase in average power but the OBI alreadyaffects the signal to noise ratio.

When the receiver detects and OBI event, it may signal the returntransmitters via a forward communication. It may have a transmitterbuilt in to provide this forward communication, or another part of thesystem may provide this forward communication.

FIG. 9 illustrates an RFoG system in which a return receiver determinesif an OBI event has occurred and provides an OBI detection signal to anRF signal generator that modulates a forward path laser. The RF signalgenerator modulates a laser which is added into the forward path with acoupler CPL. The signal is received by the end users (RTx). An RTxdetects the RF signal for OBI occurrences and controls a wavelengthshifter for the return lasers.

The signal provided to the return transmitters may, for example,indicate that an OBI event has occurred, and furthermore indicate agroup id for the receivers receiving those transmitters, and/or instructa return transmitter to change its wavelength to a particular value.

In one embodiment, a system component (e.g., the return receiver) hasknowledge of the transmitters in the system, and their status. The RFoGsystem may provide two-way communication between return transmitters andreturn receivers. This may be done using low-cost RF transmitters. Inone embodiment, the message that an OBI event has occurred is notdirected at any particular return transmitter. When the returntransmitters receive this message they may respond, for instance byshifting their wavelength, or they may not respond. Transmitters thatwere not active in a time interval that could have contributed to theOBI event do not respond, because the OBI event is not related to theiractivity. Transmitters active in the relevant interval before the OBIevent may respond by shifting their wavelength. At least twotransmitters must be active to produce an OBI event; thus the detectionsignal may result in at least two transmitters shifting wavelength,where only one needed to shift wavelength to prevent a next collisionbetween those two transmitters. In one embodiment, not all involvedtransmitters shift wavelength at each detected OBI event.

FIG. 10 is an illustration of an embodiment of logic flow for a returntransmitter determining whether or not to make a wavelength shift inresponse to an OBI event. The logic may be carried out by a customerdevice RTx such as the exemplary one illustrated in FIG. 11. Thetransmitter response may depend upon its previous activity, and on arandom selection of whether to respond or not. The magnitude of thetransmitter response in terms of wavelength shift may be deterministic(such as cycling through available wavelengths), or random, by picking anew wavelength shift. To maintain a low-cost for the returntransmitters, accurate wavelength control may not be employed. Instead,thermal control of the lasers may be applied to offset the laserwavelength in a limited range. Instead of controlling the wavelength, anamount of offset may be controlled. Environment temperature also causeslaser wavelengths to shift. Environmentally induced wavelength shift maybe compensated for, or just ignored. If ignored, the resultingwavelength drift of the laser may result in renewed OBI events withother sources. The system reduces the occurrence of these renewed OBIevents by automatic re-distribution of wavelengths when the OBI eventsoccur, keeping the rate of OBI events near zero at all times.

FIG. 12 illustrates an example of simulated OBI occurrences fordifferent parameters such as channel activity, number of users, andgroup sizes. FIG. 12 shows how the rate of OBI occurrences dropsexponentially in time until no OBI at all occurs any longer. When anywavelength has drifted significantly, the OBI increases again to a lowlevel, then decays again to zero. Temporary OBI occurrence rates under1% typically do not impact system performance significantly.

The signal provided to the return transmitters may be an RF signalmodulated onto a forward laser that is combined with the forward opticalsignal from upstream that carries programming, data, voice, and so on.The return receivers may be co-located with the return transmitters. Aforward laser may be installed in each return receiver, or one centralforward laser may be shared by many return receivers. With a sharedforward laser, an OBI event in any return receiver group may lead toun-necessary transmitter wavelength shifts in other return receivergroups, making it more difficult for each group to stabilize to an OBIfree wavelength allocation. The rate of convergence to a stableallocation depends on statistics of use and how the return transmittersrespond to the OBI event signaling.

A return receiver may embed its ID in the OBI event signaling to thereturn transmitters.

The return transmitters may make their response to the OBI eventdependent on that ID. Unnecessary wavelength shifts may thus be avoidedand the system made more stable. Return transmitters may acquire the IDof the return receiver that they are coupled to in various ways. Forexample, they may be configured with this ID upon installation. They maybe trained to this ID after installation. The first example issusceptible to installation errors. The second example prevents sucherrors but may require system interruption to re-train the system IDswhen new return transmitters are added. A third option is to let thereturn transmitters learn their group ID automatically from the forwardsignals. An OBI event is detected and the signal is provided to thereturn transmitters. In most cases there are not multiple OBI events indifferent groups that occur at the same time. The transmitters recentlyactive are the only transmitters that respond to the OBI event, andthese transmitters read the group ID in the OBI event. That group ID ismostly applicable for the transmitters in question. The transmittersthus keep track of the group ID associated with OBI event signals anddecide what group they belong to based on the majority of the ID's thatcorrelate to their activity. This self-discovery may be enhanced bydefining another forward signal; this signal may indicate a returnreceiver has detected activity in its group. The receiver may generate aconfirmation signal carrying its ID, transmitted at a time that does notcollide with such a confirmation signal from another return receiver.All the return transmitters frequently receive such confirmation signalswhenever they are active, speeding their ability to discover what groupthey belong to.

FIG. 13 illustrates an embodiment of logic flow, by way of which returntransmitters learn their associated group ID.

FIG. 14 illustrates an example of how forward signal injection may beaccomplished in the return receiver itself. FIG. 15 illustrates anembodiment in which the OBI detection signal is injected into a mainforward signal before the main forward signal is distributed overdifferent return receiver groups. FIG. 16 illustrates an embodiment inwhich the OBI forward signal is provided as an RF signal to a forwardoptical transmitter that is providing the main forward signals of theRFoG system.

The forward signal injection may be implemented in different ways. Onetechnique (FIG. 14) involves injection in the return receiver itself.The fiber to a group of homes may be coupled to a return receiveroptical port. The receiver comprises a red-blue combiner to separate theforward and return wavelengths. It has a second optical port for inputof the forward wavelength, typically 1550 nm. It also incorporates alow-power forward DFB laser that is coupled into the forward path. A WDMcoupler may be used when the wavelength of the DFB laser is sufficientlydifferent from the main forward path wavelength, or a regular couplermay be used when these wavelengths are close. A regular coupler isselected to have a low loss on the main forward path and a high loss forthe local DFB laser. The wavelength of the local DFB laser may be chosensufficiently different from the main forward path wavelength to preventOBI between these forward signals. A high loss on the forward laser,even a low power laser, is acceptable because the forward signal fromthe return receiver only carries a single channel with a low data rate.Thus a very low signal power is sufficient to detect the injectedforward signal. One advantage of this implementation is simplicity; itdoes not require communication between return receiver and othercomponents in the head end.

In another implementation (FIG. 15) the forward signal indicating OBIdetection is injected in the main forward signal before the main forwardsignal is distributed over the different return receiver groups. Thisinvolves fewer forward DFB lasers. It also involves communicationbetween the return receivers and the forward DFB laser, which may behoused in a separate transmitter unit. That transmitter unit maycomprise two optical ports, an input for the main forward optical signaland an output for the combined main forward signal and DFB. It may alsocomprise more optical ports in case this function is combined with anoptical combiner/splitter.

In a third implementation a forward OBI detection signal is provided asan RF signal to a forward optical transmitter that is also providing themain forward signals of the RFoG system (FIG. 16). The forward OBIdetection signal is distributed over a large number of return receivergroups and group IDs gain importance. This implementation may requiremore time to settle to OBI-free operation across all groups, but it alsomay provide the lowest cost. This implementation may include a centralunit that collects all OBI and activity detection signals from thedifferent return receivers and creates the forward signals.

When two-way communication is used, the return transmitters may embedinformation about status and history into the return signal that may becoupled at the headend or hub to create lookup tables and wavelengthallocation plans. This information may be embedded upon request or at(optionally randomly) spaced time intervals. In the case of malicioususe of return transmitters to create OBI interference, this informationmay be used to shut down certain return transmitters. The returntransmitters may also request confirmation signals to be embedded in theforward path to determine what return receiver group they belong to. Thestatus information provided by the return transmitters may include theamount of wavelength shift applied and the actual operating temperature.A central processing unit may use this information to derive optimumwavelength shift settings.

These techniques may also be applied to a WDM RFoG system as illustratedfor example in FIG. 17. At a headend multiple forward transmittersprovide signals to a WDM combiner; the signals are amplified anddistributed to hubs that take one or more forward wavelengths anddistribute these to end users. Optionally there may be opticalamplification in the hubs. The return traffic is received in the hub andthe output of the receivers is modulated and sent back to the headend onone or more WDM return transmitters. OBI within each group or withinmultiple groups may be managed by the receivers or by injecting aforward OBI detection signal at any other location in the forward pathas discussed in the previous section.

Return Laser Spectrum Control

OBI may be prevented if each return transmitter is allocated a specificwavelength. This may be accomplished with DWDM DFB lasers, but iscost-prohibitive. An alternative is to set each return laser to cover awide spectrum, so that OBI occurs but the resulting noise generated dueto OBI is spread over a wide spectrum such that the impact on the RFspectrum is low. This may for instance be accomplished usingwide-spectrum sources such as super-luminescent diodes (SLD). Anothersolution employs reflective SOA's at the locations of the returntransmitters. Such SOA's require a seed signal as input and vary thedegree of amplification to modulate the reflected return signal. Theseed signal may be a broadband source, spreading the OBI signal spectrumlike for SLDs. It may also be a broadband source that is provided to aDWDM filter that sends different slices of the spectrum to each returnSOA such that each return signal is a slightly different opticalfrequency, and OBI is prevented altogether. A narrowband source may beemployed, in which case various return signals are amplified versions ofthe same source. When they interfere (to create OBI) the interferencesignal is at a low frequency determined by the line-width of thenarrowband source. Modulation with the SOA's adds amplitude modulationand phase modulation to the interfering signals such that theinterference signal still possesses high frequency components. The lowfrequency interference signal may lead to fading of the received signal.If the seed signal is a pulsed signal and the pulses are short, thesignals from the individual return transmitters are separated in time,unless their optical delay is identical. In most cases this temporalseparation will prevent OBI. Application to analog signals iscomplicated by the non-linear response of SOA gain. An SOA is driven bya pre-distorted signal to compensate for non-linearity, and optionally acoupler with a feedback path is added to control the predistorter foroptimum performance. See FIG. 18 for an illustration of an exemplarysystem with these features.

Optical Signal Combiner with Reduced Optical Beat Interference

In fiber-optic communication, a single-mode optical fiber (SMF)(monomode optical fiber, single-mode optical waveguide, or unimodefiber) is an optical fiber designed to carry only a single ray of light(mode). Modes are the possible solutions of Helmholtz equation forwaves, which is obtained by combining Maxwell's equations and theboundary conditions. These modes define the way the wave travels throughspace, i.e. how the wave is distributed in space. Waves can have thesame mode but have different frequencies. This is the case insingle-mode fibers, where we can have waves with different frequencies,but of the same mode, which means that they are distributed in space inthe same way, and that gives us a single ray of light. Although the raytravels parallel to the length of the fiber, it is often calledtransverse mode since its electromagnetic vibrations occur perpendicular(transverse) to the length of the fiber.

Multi-mode fiber has higher “light-gathering” capacity than single-modeoptical fiber. In practical terms, a larger core size simplifiesconnections and also allows the use of lower-cost electronics such aslight-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers(VCSELs) which operate at the 850 nm and 1300 nm wavelength (single-modefibers used in telecommunications operate at 1310 or 1550 nm and requiremore expensive laser sources. Single mode fibers exist for nearly allvisible wavelengths of light). However, compared to single-mode fibers,the multi-mode fiber bandwidth-distance product limit is lower. Becausemulti-mode fiber has a larger core-size than single-mode fiber, itsupports more than one propagation mode; hence it is limited by modaldispersion, while single mode is not. The LED light sources sometimesused with multi-mode fiber produce a range of wavelengths and these eachpropagate at different speeds. In contrast, the lasers used to drivesingle-mode fibers produce coherent light of a single wavelength. Thischromatic dispersion is another limit to the useful length formulti-mode fiber optic cable. Because of their larger core size,multi-mode fibers have higher numerical apertures which means they arebetter at collecting light than single-mode fibers. Due to the modaldispersion in the fiber, multi-mode fiber has higher pulse spreadingrates than single mode fiber, limiting multi-mode fiber's informationtransmission capacity.

Optical signals may be combined on single mode fibers using couplers asshown in the top two combiners in FIG. 19. A typical single mode couplerthat combines two inputs has a loss of around 3 dB. This loss isfundamental to such couplers and arises from the fact that such acoupler may be considered to have two output arms. Power is coupled toeach output arm; the electrical field of the optical signal in outputarms O1 and O2 can be described by a simplified formula:

O1(t)=I1(t)+I2(t−dt)

O2(t)=I1(t)−I2(t−dt)

where I1 and I2 are the input signal electrical fields. Usually I1 andI2 are at different angular frequencies w1 and w2 respectively and havedifferent amplitudes A1 and A2respectively. The optical fields havephase noise n1(t) and n2(t) respectively.

I1(t)=A1(t)*cos(w1*t+n1(t))

I2(t)=A2(t)*cos(w2*t+n2(t))

The output fields of the coupler arms then are:

O1(t)=A1(t)*cos(w1*t+n1(t))+A2(t)*cos(w2*t+n2(t))

O2(t)=A1(t)*cos(w1*t+n1(t))−A2(t)*cos(w2*t+n2(t))

When measuring the output powers P1 and P2 of the arms with a detector,the following result is obtained for O1:

P1(t)=O1(t)̂2=(A1(t)*cos(w1*t+n1(t))+A2(t)*cos(w2*t+n2(t)))̂2=A1(t)̂2*cos(w1*t+n1(t))̂2+2*A1(t)*A2(t)*cos(w1*t+n1(t))*A2(t)*cos(w2*t+n2(t))+A2(t)̂2*cos(w2*t+n2(t)̂2=A1(t)̂2*(0.5+0.5*cos(2*(w1*t+n1(t)))+A2(t)̂2*(0.5+0.5*cos(2*(w2*t+n2(t)))+A1(t)*A2(t)*[cos(w1*t+n1(t)+w2*t+n2(t))+cos(w1*t+n1(t)−w2*t−n2(t))]

RF detectors have a limited RF response. In many practical CATV systemsthis response is on the order of 1 GHz. The optical frequencies may bein the 100 THz range, so that the terms with 2w1, 2w2 or w1+w2 angularfrequencies do not result in a detectable current. The result can thusbe simplified to:

P1(t)=0.5*[A1(t)̂2+A2(t)̂2]+A1(t)*A2(t)*cos(w1*t+n1(t)−w2*t−n2(t))

The first two terms represent the input powers to the coupler that areadded up and divided by 2 (the 3 dB loss). The last term results fromthe interaction of the optical fields of the two inputs. In many casesthe optical frequencies of the two inputs differ by more than 1 GHz. Inthis case the third term will not result in a detectable photocurrent.In some cases however the optical frequencies are very close, such thatthe third term results in a detector signal at the differencefrequencies that is well within the range of normal RF frequencies. Inreturn systems, for instance, this is frequencies below 50 MHz. In thesesituations the detector output comprises the wanted addition of the twoinputs, plus an additional strong signal due to the interaction of thetwo inputs.

If the inputs had no phase noise (n1=0, n2=0), this additional signalwould be a tone at angular frequency w1-w2. In practice, optical sourceshave a significant phase noise such that the additional signal is anoise band around a center frequency w1-w2. If the frequency range ofthis unwanted signal band overlaps with wanted signals, the signal tonoise ratio of the wanted signal may be severely impacted by OBI.

If the input signals are detected separately, the interaction term doesnot exist and there is no optical beat interference. Input power peroptical signal in this case is a factor of two higher, potentiallyresulting in better signal to noise performance. Separate detectioninvolves one detector and subsequent amplifier circuit per input, andthus can be costly for a large number of inputs.

The signal out of a detector at the second output is given by:

P2(t)=0.5*[A1(t)̂2+A2(t)̂2]−A1(t)*A2(t)*cos(w1*t+n1(t)−w2*t−n2(t))

This is nearly the same expression as for the signal out of the firstoutput detector, except for the sign difference in front of the unwantedterm. Thus the sum of the detectors:

P1(t)+P2(t)=A1(t)̂2+A2(t)̂2

has the full signal power and is free of optical beat interference. Adetector could be provided at each coupler output arm to avoid theoptical beat interference, but that would again be costly. The reasonthat the sum of the outputs of the coupler is free of optical beatinterference is that every photon that enters the coupler will exit iton one of the two arms. When the output photons of both arms are counted(same as measuring the output power of both arms) the total measuredpower becomes the sum of the input powers. Vice versa, the inherent 3 dBloss of the single mode optical coupler enables optical beatinterference in the first place when, as usual, only one output is used.

Multi mode couplers differ from single mode couplers in that theycombine multiple input signals with a very low loss. Multimode couplersexcite different output modes in the output arm of the coupler for eachoptical input signal (something that is not possible in a single modedevice that can only output power in a single mode per arm). Light maybe coupled from single mode to multimode fibers with a low loss; thesingle mode input will excite a matched mode pattern in the multimodewaveguide. However, coupling light from a multimode fiber back into asingle mode fiber is difficult, because only one of many possible modalpatterns couples well with the single mode fiber. A system with multiplesingle mode fiber inputs each carrying an optical signal may beconverted to a multimode signal, and with a multimode coupler beprovided to a multimode output. Due to the low loss in this conversionprocess any optical beat interference is usually low.

In another formulation; the different inputs excite different modalpatterns in the multimode output that do not mutually interfere. If theoutput of the multimode fiber is coupled to a photo detector, thedetector measures the sum of all input powers without optical beatinterference as long as the detector is large enough to capture thecomplete output beam of the multimode fiber. Typical multimode fibershave core diameters in the order of 50-80 um. Typical detector diametersfor GHz range detectors are on the order of 50 um. Lower frequencydetectors can be larger, for instance for systems with response up to 50or 100 MHz.

FIG. 19 illustrates a multimode coupler that may be used to combinemultiple single mode fiber inputs with optical frequencies sufficientlyclose that optical beat interference could occur. This results insuppressed optical beat interference and low loss for the individualinputs. The coupler output is provided to a multimode detector andamplifier to recover RF signals.

Many multimode (MM) couplers are not lossless, so not every MM couplerwill prevent OBI. However MM couplers do exist that are lossless inprinciple and these may be utilized as a MM coupler that prevents OBI.

In one embodiment, a headend has multiple outgoing fibers, each fiberproviding a return signal to the headend, and these return signals needto be combined before being applied to an optical receiver. The use of asingle mode (SM) combiner causes OBI. The use of a MM coupler may reduceor prevent OBI. Alternately, a detector may be used with multiple SMinputs that are imaged on a MM detector, for instance using differentbeam angles.

In another application, an RFoG system utilizes a single mode fiberrunning from a headend to a node; from the node the signals aredistributed to the end user devices over relatively short links (e.g.less than half the link distance from the headend to the node).Distribution may be done with MM fiber and MM couplers. MM fiber cannotsupport the distribution of forward signals over long distances, but onshort drops it can be adequate. Return signals are injected into the MMfibers and get combined by the same coupler that distributes the forwardsignal. For a coupler of the right type this combination may be low-losswhile the forward split may distribute power evenly over the outputports. The low loss combiner is OBI free and the combined return signalis detected after the combiner.

In another application, an RFoG system comprises a headend and a firstsplit into N output fibers that are then split in N nodes into a number(for instance M) of output fibers per node. At each node the M outgoingfibers carry forward and return traffic. The return traffic is extractedoff each of the M fibers and combined in a MM coupler connected to a MMfiber. The MM fiber is then coupled to a MM detector. Alternately, theMM coupler has an integrated MM detector. The detector output isamplified and provided to a laser that sends the return traffic back tothe headend.

The transmission back to the headend may be on a separate fiber whichmay be SM or MM, or it may be on the headend to node fiber that carriesthe forward traffic. In case the fiber back to the headend is a separateMM fiber, it may also be coupled to the MM coupler in the node (suchthat detection and re-transmission is not needed). A MM detector (orfurther MM couplers) may then be used in the headend. In case the fiberback to the headend is SM (a separate SM fiber or shared with the SMfiber from the headend) the return laser transmission into that fibermay use a WDM laser (CWDM or DWDM) such that the return transmissionfrom each node is at a different wavelength. This allows combination ofthe N return signals at the headend in the optical domain without OBI.

If a first broadband source is used with a bandwidth of several 100 GHz,the first broadband source may be combined with a second broadbandsource in a coupler. OBI will occur at the coupler output. However, dueto the large bandwidth of the sources the OBI will be spread overseveral 100 GHz to a few THz. The amount of OBI noise that can thus fallinto an RF channel of 4 MHz width is reduced by a factor on the order of4 MHz/1 THz or 10*log(4*10̂−6) dB (54 dB).

Thus the impact of OBI is effectively eliminated. A broadband source maybe generated for instance with the use of optical amplifiers and opticalfilters. With an optical splitter the output of the broadband source maybe distributed over a number of output fibers that are terminated inreceivers with reflective amplifiers. Modulation of the gain of thereflective amplifiers modulates the reflected light intensity of thebroadband source. The reflective amplifier outputs are thus combined bythe same splitter that was used to distribute the broadband sourceoutput. The combined return signal from the splitter will suffer fromOBI but this OBI is spread out over such a wide frequency range that itdoes no longer significantly affect the performance of the system.

Bias Overshoot For Pre-Steady State Laser

Lasers used in RFoG applications are generally either 1310 or 1610uncooled DFB lasers. The wavelength of such lasers may shift at the rateof approximately 0.1 nm/C. When a DFB laser is turned on, because of thepresence of the RF at its input according to the DOCSIS/RFoG turnon/turn off algorithm, the laser begins at the ambient temperature andtakes some amount of time until it reaches its steady state temperaturesubject to the mass of the laser chip and its holding mechanism. Duringthe time that the laser is achieving its steady state temperature,assuming the laser is turned on for the duration of this time, the laserwavelength is continually moving.

EXAMPLE: As an example if the ambient temperature were OC and the steadystate temperature of the laser happened to be 25C, the temperature movesthru 25C which is a consequent movement of 2.5 nm of the laserwavelength. This substantial movement of the laser wavelength causes thelaser to OBI with other lasers that are turned on at the same time andresults in a more substantial OBI effect. OBI occurs when the two ormore wavelengths are within 0.0125 nm of each other.

Let the temperature coefficient of the laser be represented by xx.Typically xx is on the order of 0.08 nm/degree C. The power dissipatedin the laser is a function of current. To a first approximation it islinear in the laser current and the power is Pdiss=yy*Ilaser. Typicallyyy is on the order of 2V (Ilaser in A, Pdiss in Watt).

The thermal resistance of a laser chip mounted in a package to a firstapproximation is zz (typically zz=100 K/W). The steady state temperatureincrease of a laser due to current isdeltaT_steady=Pdiss*zz=yy*zz*Ilaser and the steady state wavelengthshift is delta_lambda_steady=xx*yy*zz*Ilaser. Due to the effectivethermal mass of the chip this wavelength shift is not reachedinstantaneously after turn on. The steady state condition is approachedwith a function: delat_lambda(t)=delta_lambda_steady*f(t) where f(t) istypically an exponential-like function such as f(t)=1-exp(−t/tau) withtau on the order of 1-10 usec. When Ilaser is not constant the relationis more complicated. During a time period that Ilaser is set higher thansteady state the temperature rise and thus the wavelength shift isaccelerated.

The laser's bias current be “overshot” and depleted to normal levels tospeed up the process of laser temperature change and by extension thewavelength. During the overshoot the steady state described above isreached. For the duration of the overshoot, even though the bias currentand by extension the output power of the laser and by extension theinput power to the receiver has increased, since the RF level going tothe laser is helped constant, there is a consequent decrease in thelaser OMI. This lower OMI is in proportion to the increase in receivedoptical power and therefore has the effect of maintaining the RF levelat the receiver hinged to a constant value.

The back facet photodiode monitor of the laser is a pn junction devicewhen it is not reverse biased as indeed any photodiode is when notbiased similarly. Therefore when this back facet monitor is observedwithout the reverse bias, it provides an accurate estimation oftemperature inside the laser chamber, providing a measure of thetemperature inside the laser chamber even though the RFoG laserstraditional would not have a thermistor.

A map of the steady state temperature of each laser may be created bypolling the back facet monitor of each laser in the RFoG set. With anappropriate overshoot or undershooting of the bias current as the casemaybe, there is further reduction in the OBI.

Implementations and Alternatives

Those having skill in the art will appreciate that there are variouslogic implementations by which processes and/or systems described hereincan be effected (e.g., hardware, software, and/or firmware), and thatthe preferred vehicle will vary with the context in which the processesare deployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a hardware and/orfirmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a solely software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes described herein may be effected, none of which isinherently superior to the other in that any vehicle to be utilized is achoice dependent upon the context in which the vehicle will be deployedand the specific concerns (e.g., speed, flexibility, or predictability)of the implementer, any of which may vary. Those skilled in the art willrecognize that optical aspects of implementations may involveoptically-oriented hardware, software, and or firmware.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood as notorious by those within the art that each functionand/or operation within such block diagrams, flowcharts, or examples canbe implemented, individually and/or collectively, by a wide range ofhardware, software, firmware, or virtually any combination thereof.Several portions of the subject matter described herein may beimplemented via Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs), digital signal processors (DSPs), orother integrated formats. However, those skilled in the art willrecognize that some aspects of the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in standard integratedcircuits, as one or more computer programs running on one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs running on one or more processors(e.g., as one or more programs running on one or more microprocessors),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and/or firmwarewould be well within the skill of one of skill in the art in light ofthis disclosure. In addition, those skilled in the art will appreciatethat the mechanisms of the subject matter described herein are capableof being distributed as a program product in a variety of forms, andthat an illustrative embodiment of the subject matter described hereinapplies equally regardless of the particular type of signal bearingmedia used to actually carry out the distribution. Examples of a signalbearing media include, but are not limited to, the following: recordabletype media such as floppy disks, hard disk drives, CD ROMs, digitaltape, and computer memory; and transmission type media such as digitaland analog communication links using TDM or IP based communication links(e.g., packet links).

In a general sense, those skilled in the art will recognize that thevarious aspects described herein which can be implemented, individuallyand/or collectively, by a wide range of hardware, software, firmware, orany combination thereof can be viewed as being composed of various typesof “electrical circuitry.” Consequently, as used herein “electricalcircuitry” includes, but is not limited to, electrical circuitry havingat least one discrete electrical circuit, electrical circuitry having atleast one integrated circuit, electrical circuitry having at least oneapplication specific integrated circuit, electrical circuitry forming ageneral purpose computing device configured by a computer program (e.g.,a general purpose computer configured by a computer program which atleast partially carries out processes and/or devices described herein,or a microprocessor configured by a computer program which at leastpartially carries out processes and/or devices described herein),electrical circuitry forming a memory device (e.g., forms of randomaccess memory), and/or electrical circuitry forming a communicationsdevice (e.g., a modem, communications switch, or optical-electricalequipment).

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use standard engineering practices to integrate suchdescribed devices and/or processes into larger systems. That is, atleast a portion of the devices and/or processes described herein can beintegrated into a network processing system via a reasonable amount ofexperimentation.

The foregoing described aspects depict different components containedwithin, or connected with, different other components. It is to beunderstood that such depicted architectures are merely exemplary, andthat in fact many other architectures can be implemented which achievethe same functionality. In a conceptual sense, any arrangement ofcomponents to achieve the same functionality is effectively “associated”such that the desired functionality is achieved. Hence, any twocomponents herein combined to achieve a particular functionality can beseen as “associated with” each other such that the desired functionalityis achieved, irrespective of architectures or intermedial components.Likewise, any two components so associated can also be viewed as being“operably connected”, or “operably coupled”, to each other to achievethe desired functionality.

What is claimed is:
 1. A bi-directional optical transceiver, comprising:a plurality of single mode optical ports; a multi-mode optical port; amulti-mode optical combiner coupled to the plurality of optical portsand configured to combine a plurality of single mode optical signalsreceived at the plurality of single mode optical ports into a multi-modeoptical signal at the multi-mode optical port, each single mode opticalsignal having a distinct optical mode that does not interfere with theoptical mode of the other single mode optical signals; a photo detectorcoupled to the multi-mode optical combiner and formed to detect a totaloptical power of the plurality of single mode optical signals in themulti-mode optical signal; and an amplifier coupled to receive an outputof the photo detector.
 2. An optical receiver, comprising: a pluralityof single mode optical ports; a multi-mode photo detector coupled toreceive a plurality of single mode optical signals from each of thesingle mode optical ports, each single mode optical signal having adistinct optical mode that does not interfere with the optical mode ofthe other single mode optical signals, the optical combiner formed todetect a total optical power of the plurality of single mode opticalsignals; and the coupling of single mode optical ports and the photodetector arranged to image each of the plurality of single mode opticalsignals at a different beam angle from the others.
 3. A radio frequencyover glass (RFoG) communication system, comprising: a plurality ofsingle mode optical fibers coupled between a digital contentdistribution headend and a regional distribution node; a plurality ofmulti-mode optical fibers between the regional distribution node and aplurality of customer sites, each multi-mode optical fiber less thanhalf a length of any one of the single mode optical fibers; a pluralityof multi-mode optical couplers, each formed and arranged to couple aplurality of single mode optical signals from the plurality of singlemode optical fibers to one of the multi mode optical fibers, each singlemode optical signal having a distinct optical mode that does notinterfere with the optical mode of the other single mode opticalsignals; a photo detector coupled to each multi-mode optical fiber andformed to detect a total optical power of a multi mode signal from amulti-mode optical fiber it is coupled with; and an amplifier coupled toreceive an output of each photo detector.